Accumulating evidence indicates that many human tumors, including breast cancer, express tumor-specific antigens, but that the expression of these "foreign" proteins on the surface of these malignant cells fails to induce an adequate immune response. Nonetheless, in vitro and in vivo studies using isolated and stimulated tumor infiltrating lymphocytes, have indicated that immunological enhancing strategies can lead to effective anti-tumor immunity. Similarly, tumor cell lines transfected with lymphokine genes such as IL-2 or IL-4 have been rendered highly immunogenic. Thus, the failure of lymphocytes to reject endogenous tumors may be related to the inability of the malignant cells to activate helper T cells to produce cytokines leading to the expansion and activation of cytotoxic T cells and other immune effector populations. The productive stimulation of T cells requires two signals. The first is antigen-specific and results from engagement of the T cell receptor. This second signal can be delivered by binding of the T cell accessory molecule CD28 to its ligand B7 expressed on the surface of activated antigen presenting cells. Additional studies have shown that failure to deliver a second signal (i.e., stimulation of the T cell receptor alone) can lead to a T cell anergy. Indeed, blockade of B7-mediated T cell stimulation can lead to long-lasting organ and tissue transplant survival. Thus the lack of expression of B7 on non-immune cells may represent a normal mechanism of immunologic tolerance to self-antigens, a mechanism which may be undesirable when cells undergo neoplastic transformation and express tumor antigens. The goals of this proposal are to examine the efficacy of B7 gene transfer in inducing an in vitro and in vivo immune response to breast cancer. Preliminary experiments using low stringency screening of a rat splenocyte cDNA library with a murine B7 probe have yielded several positive clones. In specific aim 1, we will complete these initial experiments, and clone a full length rat B7 cDNA. In specific aim 2, we will transfect rat B7 into previously established rat primary breast carcinoma cell lines, and test the ability of this maneuver to induce anti-tumor immunity when these cells are transplanted into otherwise untreated syngeneic recipients. If B7 gene transfer is effective, these studies will also determine whether B7+ breast cancer cells can induce a response against non-transfected cells when the latter are transplanted at the same time or at a later point. Parallel in vitro studies will test the ability of B7+ cells to induce a proliferative or cytotoxic T cell response. In specific aim 3, we will transfect human B7 (previously cloned) into human mammary carcinoma cell lines which we have previously derived, and determine whether this is able to induce an in vitro immune response in autologous lymphocytes. In the last specific aim, we will seek to develop reliable methods to derive cell lines from human breast cancers, since if B7 gene transfer proves effective, then ultimate application of this as a human therapeutic tool will require the capability of deriving cell lines from breast cancer patients.